A conventional invert emulsion fluid for drilling in oil and gas exploration generally includes clay in the formulation, where the clay acts as the primary rheology (viscosity) modifier. The clay can be organophilic clay or organoclay. Drilling fluids formulated with an organophilic clay can have a degradation of rheology properties over time. In other words, the organophilic clay formulated fluids can have a loss of viscosity over time, owing to the inability of the clay to maintain a necessary level of viscosity. One solution to the loss of viscosity with time is to add an excess amount of an organophilic clay to the drilling fluid, or to add an excess of low gravity solids (LGS) to the drilling fluid, or both. However, adding excess clay, or LGS, or both can increase the cost of drilling and can severely impact other important drilling fluid properties. These impacts on cost, or fluid property, or both can necessitate further costly treatments downhole or to the drilling fluid as remedies. For example, the addition of excess LGS can increase the plastic viscosity and the solids volume percentage, which can impact the rate of penetration of a drill bit into a formation, thus increasing the cost of drilling.
Drilling fluid rheology can change with increasing well depth as a result of changes in pressure and temperature. Such changes can cause changes in the equivalent circulating density (ECD) while drilling advances down through a formation. These fluctuations in ECD can lead to fracture of the formation when operating in a narrow window of pore pressure and fracture gradient. This can lead to formation damage and mud losses, thus increasing drilling costs. The use of thinner fluids to minimize rheology fluctuations, in general, can lead to lesser ECD; however, the fluid rheology may need to be such that the fluid properties enable cuttings removal and help to suspend drill solids. There are competing needs of greater viscosity for cuttings removal and suspension of solids versus lesser viscosity for better ECD.